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  MyBatis-多对一,一对多处理
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      <h1 id="MyBatis-多对一-一对多处理"><a href="#MyBatis-多对一-一对多处理" class="headerlink" title="MyBatis 多对一,一对多处理"></a>MyBatis 多对一,一对多处理</h1><h2 id="多对一的处理"><a href="#多对一的处理" class="headerlink" title="多对一的处理"></a>多对一的处理</h2><p>多对一的理解：</p>
<ul>
<li>多个学生对应一个老师</li>
<li>如果对于学生这边，就是一个多对一的理解</li>
<li>也就是从学生这边关联了一个老师</li>
</ul>
<a id="more"></a>

<p><strong>数据库设计（环境搭建）</strong></p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br></pre></td><td class="code"><pre><span class="line">USE mybatis;</span><br><span class="line">CREATE TABLE &#96;teacher&#96; (</span><br><span class="line">  &#96;id&#96; INT(10) NOT NULL,</span><br><span class="line">  &#96;name&#96; VARCHAR(30) DEFAULT NULL,</span><br><span class="line">  PRIMARY KEY (&#96;id&#96;)</span><br><span class="line">) ENGINE&#x3D;INNODB DEFAULT CHARSET&#x3D;utf8</span><br><span class="line"></span><br><span class="line">INSERT INTO teacher(&#96;id&#96;, &#96;name&#96;) VALUES (1, &#39;朱老师&#39;); </span><br><span class="line"></span><br><span class="line">CREATE TABLE &#96;student&#96; (</span><br><span class="line">  &#96;id&#96; INT(10) NOT NULL,</span><br><span class="line">  &#96;name&#96; VARCHAR(30) DEFAULT NULL,</span><br><span class="line">  &#96;tid&#96; INT(10) DEFAULT NULL,</span><br><span class="line">  PRIMARY KEY (&#96;id&#96;),</span><br><span class="line">  KEY &#96;fktid&#96; (&#96;tid&#96;),</span><br><span class="line">  CONSTRAINT &#96;fktid&#96; FOREIGN KEY (&#96;tid&#96;) REFERENCES &#96;teacher&#96; (&#96;id&#96;)</span><br><span class="line">) ENGINE&#x3D;INNODB DEFAULT CHARSET&#x3D;utf8</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">INSERT INTO &#96;student&#96; (&#96;id&#96;, &#96;name&#96;, &#96;tid&#96;) VALUES (&#39;1&#39;, &#39;小明&#39;, &#39;1&#39;); </span><br><span class="line">INSERT INTO &#96;student&#96; (&#96;id&#96;, &#96;name&#96;, &#96;tid&#96;) VALUES (&#39;2&#39;, &#39;小红&#39;, &#39;1&#39;); </span><br><span class="line">INSERT INTO &#96;student&#96; (&#96;id&#96;, &#96;name&#96;, &#96;tid&#96;) VALUES (&#39;3&#39;, &#39;小张&#39;, &#39;1&#39;); </span><br><span class="line">INSERT INTO &#96;student&#96; (&#96;id&#96;, &#96;name&#96;, &#96;tid&#96;) VALUES (&#39;4&#39;, &#39;小李&#39;, &#39;1&#39;); </span><br><span class="line">INSERT INTO &#96;student&#96; (&#96;id&#96;, &#96;name&#96;, &#96;tid&#96;) VALUES (&#39;5&#39;, &#39;小王&#39;, &#39;1&#39;);</span><br></pre></td></tr></table></figure>

<p><img src="https://zhuuu-bucket.oss-cn-beijing.aliyuncs.com/img/20200217115725.png" alt=""></p>
<h3 id="搭建测试环境"><a href="#搭建测试环境" class="headerlink" title="搭建测试环境"></a>搭建测试环境</h3><h3 id="Lombok的使用"><a href="#Lombok的使用" class="headerlink" title="Lombok的使用"></a>Lombok的使用</h3><ol>
<li>IDEA安装Lombok插件【在Plugins中安装】</li>
<li>引入maven依赖</li>
</ol>
<figure class="highlight xml"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">&lt;!-- https://mvnrepository.com/artifact/org.projectlombok/lombok --&gt;</span></span><br><span class="line"><span class="tag">&lt;<span class="name">dependency</span>&gt;</span></span><br><span class="line">  <span class="tag">&lt;<span class="name">groupId</span>&gt;</span>org.projectlombok<span class="tag">&lt;/<span class="name">groupId</span>&gt;</span></span><br><span class="line">  <span class="tag">&lt;<span class="name">artifactId</span>&gt;</span>lombok<span class="tag">&lt;/<span class="name">artifactId</span>&gt;</span></span><br><span class="line">  <span class="tag">&lt;<span class="name">version</span>&gt;</span>1.16.10<span class="tag">&lt;/<span class="name">version</span>&gt;</span></span><br><span class="line"><span class="tag">&lt;/<span class="name">dependency</span>&gt;</span></span><br></pre></td></tr></table></figure>

<ol start="3">
<li>在代码中增加注解(@Data)</li>
</ol>
<p>使用@Data前后对比</p>
<p><img src="https://zhuuu-bucket.oss-cn-beijing.aliyuncs.com/img/20200217144634.png" alt=""></p>
<p><img src="https://zhuuu-bucket.oss-cn-beijing.aliyuncs.com/img/20200217144741.png" alt=""></p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">@Data</span> <span class="comment">//GET,SET,ToString，有参，无参构造</span></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Teacher</span> </span>&#123;</span><br><span class="line">    <span class="keyword">private</span> <span class="keyword">int</span> id;</span><br><span class="line">    <span class="keyword">private</span> String name;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">@Data</span></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Student</span> </span>&#123;</span><br><span class="line">    <span class="keyword">private</span> <span class="keyword">int</span> id;</span><br><span class="line">    <span class="keyword">private</span> String name;</span><br><span class="line">    <span class="comment">//多个学生可以是同一个老师，即多对一</span></span><br><span class="line">    <span class="keyword">private</span> Teacher teacher;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<ol start="4">
<li>编写实体类对应的Mapper接口【两个】</li>
</ol>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">StudentMapper</span> </span>&#123;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">interface</span> <span class="title">TeacherMapper</span> </span>&#123;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<ol start="5">
<li>编写Mapper接口对应的mapper.xml配置文件【两个】</li>
</ol>
<figure class="highlight xml"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">&lt;?xml version="1.0" encoding="UTF-8" ?&gt;</span></span><br><span class="line"><span class="meta">&lt;!DOCTYPE <span class="meta-keyword">mapper</span></span></span><br><span class="line"><span class="meta">        <span class="meta-keyword">PUBLIC</span> <span class="meta-string">"-//mybatis.org//DTD Mapper 3.0//EN"</span></span></span><br><span class="line"><span class="meta">        <span class="meta-string">"http://mybatis.org/dtd/mybatis-3-mapper.dtd"</span>&gt;</span></span><br><span class="line"><span class="tag">&lt;<span class="name">mapper</span> <span class="attr">namespace</span>=<span class="string">"com.zhuuu.dao.TeacherMapper"</span>&gt;</span></span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="tag">&lt;/<span class="name">mapper</span>&gt;</span></span><br></pre></td></tr></table></figure>

<figure class="highlight xml"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">&lt;?xml version="1.0" encoding="UTF-8" ?&gt;</span></span><br><span class="line"><span class="meta">&lt;!DOCTYPE <span class="meta-keyword">mapper</span></span></span><br><span class="line"><span class="meta">        <span class="meta-keyword">PUBLIC</span> <span class="meta-string">"-//mybatis.org//DTD Mapper 3.0//EN"</span></span></span><br><span class="line"><span class="meta">        <span class="meta-string">"http://mybatis.org/dtd/mybatis-3-mapper.dtd"</span>&gt;</span></span><br><span class="line"><span class="tag">&lt;<span class="name">mapper</span> <span class="attr">namespace</span>=<span class="string">"com.zhuuu.dao.StudentMapper"</span>&gt;</span></span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="tag">&lt;/<span class="name">mapper</span>&gt;</span></span><br></pre></td></tr></table></figure>

<h4 id="按照查询嵌套处理"><a href="#按照查询嵌套处理" class="headerlink" title="按照查询嵌套处理"></a>按照查询嵌套处理</h4><ol>
<li>给StudentMapper接口增加方法</li>
</ol>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//获取所有学生及对应老师的信息</span></span><br><span class="line"><span class="function"><span class="keyword">public</span> List&lt;Student&gt; <span class="title">getStudents</span><span class="params">()</span></span>;</span><br></pre></td></tr></table></figure>

<ol start="2">
<li>编写对应的Mapper文件</li>
</ol>
<figure class="highlight xml"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">&lt;?xml version="1.0" encoding="UTF-8" ?&gt;</span></span><br><span class="line"><span class="meta">&lt;!DOCTYPE <span class="meta-keyword">mapper</span></span></span><br><span class="line"><span class="meta">        <span class="meta-keyword">PUBLIC</span> <span class="meta-string">"-//mybatis.org//DTD Mapper 3.0//EN"</span></span></span><br><span class="line"><span class="meta">        <span class="meta-string">"http://mybatis.org/dtd/mybatis-3-mapper.dtd"</span>&gt;</span></span><br><span class="line"><span class="tag">&lt;<span class="name">mapper</span> <span class="attr">namespace</span>=<span class="string">"com.zhuuu.mapper.StudentMapper"</span>&gt;</span></span><br><span class="line"></span><br><span class="line">    <span class="comment">&lt;!--</span></span><br><span class="line"><span class="comment">    需求：获取所有学生及对应老师的信息</span></span><br><span class="line"><span class="comment">    思路：</span></span><br><span class="line"><span class="comment">        1. 获取所有学生的信息</span></span><br><span class="line"><span class="comment">        2. 根据获取的学生信息的老师ID-&gt;获取该老师的信息</span></span><br><span class="line"><span class="comment">        3. 思考问题，这样学生的结果集中应该包含老师，该如何处理呢，数据库中我们一般使用关联查询？</span></span><br><span class="line"><span class="comment">            1. 做一个结果集映射：StudentTeacher</span></span><br><span class="line"><span class="comment">            2. StudentTeacher结果集的类型为 Student</span></span><br><span class="line"><span class="comment">            3. 学生中老师的属性为teacher，对应数据库中为tid。</span></span><br><span class="line"><span class="comment">               多个 [1,...）学生关联一个老师=&gt; 一对一，一对多</span></span><br><span class="line"><span class="comment">            4. 查看官网找到：association – 一个复杂类型的关联；使用它来处理关联查询</span></span><br><span class="line"><span class="comment">    --&gt;</span></span><br><span class="line">    <span class="tag">&lt;<span class="name">select</span> <span class="attr">id</span>=<span class="string">"getStudents"</span> <span class="attr">resultMap</span>=<span class="string">"StudentTeacher"</span>&gt;</span></span><br><span class="line">      select * from student</span><br><span class="line">    <span class="tag">&lt;/<span class="name">select</span>&gt;</span></span><br><span class="line">    <span class="tag">&lt;<span class="name">resultMap</span> <span class="attr">id</span>=<span class="string">"StudentTeacher"</span> <span class="attr">type</span>=<span class="string">"Student"</span>&gt;</span></span><br><span class="line">        <span class="comment">&lt;!--association关联属性  property属性名 javaType属性类型 column在多的一方的表中的列名--&gt;</span></span><br><span class="line">        <span class="tag">&lt;<span class="name">association</span> <span class="attr">property</span>=<span class="string">"teacher"</span>  <span class="attr">column</span>=<span class="string">"tid"</span> <span class="attr">javaType</span>=<span class="string">"Teacher"</span> <span class="attr">select</span>=<span class="string">"getTeacher"</span>/&gt;</span></span><br><span class="line">    <span class="tag">&lt;/<span class="name">resultMap</span>&gt;</span></span><br><span class="line">    <span class="comment">&lt;!--</span></span><br><span class="line"><span class="comment">    这里传递过来的id，只有一个属性的时候，下面可以写任何值</span></span><br><span class="line"><span class="comment">    association中column多参数配置：</span></span><br><span class="line"><span class="comment">        column="&#123;key=value,key=value&#125;"</span></span><br><span class="line"><span class="comment">        其实就是键值对的形式，key是传给下个sql的取值名称，value是片段一中sql查询的字段名。</span></span><br><span class="line"><span class="comment">    --&gt;</span></span><br><span class="line">    <span class="tag">&lt;<span class="name">select</span> <span class="attr">id</span>=<span class="string">"getTeacher"</span> <span class="attr">resultType</span>=<span class="string">"teacher"</span>&gt;</span></span><br><span class="line">        select * from teacher where id = #&#123;id&#125;</span><br><span class="line">    <span class="tag">&lt;/<span class="name">select</span>&gt;</span></span><br><span class="line"></span><br><span class="line"><span class="tag">&lt;/<span class="name">mapper</span>&gt;</span></span><br></pre></td></tr></table></figure>

<ol start="3">
<li><p>在配置文件中 ，注册Mapper</p>
</li>
<li><p>注意点说明</p>
</li>
</ol>
<figure class="highlight xml"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="tag">&lt;<span class="name">resultMap</span> <span class="attr">id</span>=<span class="string">"StudentTeacher"</span> <span class="attr">type</span>=<span class="string">"Student"</span>&gt;</span></span><br><span class="line">    <span class="comment">&lt;!--association关联属性  property属性名 javaType属性类型 column在多的一方的表中的列名--&gt;</span></span><br><span class="line">    <span class="tag">&lt;<span class="name">association</span> <span class="attr">property</span>=<span class="string">"teacher"</span>  <span class="attr">column</span>=<span class="string">"&#123;id=tid,name=tid&#125;"</span> <span class="attr">javaType</span>=<span class="string">"Teacher"</span> <span class="attr">select</span>=<span class="string">"getTeacher"</span>/&gt;</span></span><br><span class="line"><span class="tag">&lt;/<span class="name">resultMap</span>&gt;</span></span><br><span class="line"><span class="comment">&lt;!--</span></span><br><span class="line"><span class="comment">这里传递过来的id，只有一个属性的时候，下面可以写任何值</span></span><br><span class="line"><span class="comment">association中column多参数配置：</span></span><br><span class="line"><span class="comment">    column="&#123;key=value,key=value&#125;"</span></span><br><span class="line"><span class="comment">    其实就是键值对的形式，key是传给下个sql的取值名称，value是片段一中sql查询的字段名。</span></span><br><span class="line"><span class="comment">--&gt;</span></span><br><span class="line"><span class="tag">&lt;<span class="name">select</span> <span class="attr">id</span>=<span class="string">"getTeacher"</span> <span class="attr">resultType</span>=<span class="string">"teacher"</span>&gt;</span></span><br><span class="line">    select * from teacher where id = #&#123;id&#125; and name = #&#123;name&#125;</span><br><span class="line"><span class="tag">&lt;/<span class="name">select</span>&gt;</span></span><br></pre></td></tr></table></figure>

<ol start="5">
<li>测试</li>
</ol>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">@Test</span></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">testGetStudents</span><span class="params">()</span></span>&#123;</span><br><span class="line">    SqlSession session = MybatisUtils.getSession();</span><br><span class="line">    StudentMapper mapper = session.getMapper(StudentMapper<span class="class">.<span class="keyword">class</span>)</span>;</span><br><span class="line"></span><br><span class="line">    List&lt;Student&gt; students = mapper.getStudents();</span><br><span class="line"></span><br><span class="line">    <span class="keyword">for</span> (Student student : students)&#123;</span><br><span class="line">        System.out.println(</span><br><span class="line">                <span class="string">"学生名:"</span>+ student.getName()</span><br><span class="line">                        +<span class="string">"\t老师:"</span>+student.getTeacher().getName());</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h4 id="按照结果嵌套处理"><a href="#按照结果嵌套处理" class="headerlink" title="按照结果嵌套处理"></a>按照结果嵌套处理</h4><ol>
<li>编写接口方法</li>
</ol>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> List&lt;Student&gt; <span class="title">getStudents2</span><span class="params">()</span></span>;</span><br></pre></td></tr></table></figure>

<ol start="2">
<li>编写对应的Mapper文件</li>
</ol>
<figure class="highlight xml"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">&lt;!--</span></span><br><span class="line"><span class="comment">按查询结果嵌套处理</span></span><br><span class="line"><span class="comment">思路：</span></span><br><span class="line"><span class="comment">    1. 直接查询出结果，进行结果集的映射</span></span><br><span class="line"><span class="comment">--&gt;</span></span><br><span class="line"><span class="tag">&lt;<span class="name">select</span> <span class="attr">id</span>=<span class="string">"getStudents2"</span> <span class="attr">resultMap</span>=<span class="string">"StudentTeacher2"</span> &gt;</span></span><br><span class="line">    select s.id sid, s.name sname , t.name tname</span><br><span class="line">    from student s,teacher t</span><br><span class="line">    where s.tid = t.id</span><br><span class="line"><span class="tag">&lt;/<span class="name">select</span>&gt;</span></span><br><span class="line"></span><br><span class="line"><span class="tag">&lt;<span class="name">resultMap</span> <span class="attr">id</span>=<span class="string">"StudentTeacher2"</span> <span class="attr">type</span>=<span class="string">"Student"</span>&gt;</span></span><br><span class="line">    <span class="tag">&lt;<span class="name">id</span> <span class="attr">property</span>=<span class="string">"id"</span> <span class="attr">column</span>=<span class="string">"sid"</span>/&gt;</span></span><br><span class="line">    <span class="tag">&lt;<span class="name">result</span> <span class="attr">property</span>=<span class="string">"name"</span> <span class="attr">column</span>=<span class="string">"sname"</span>/&gt;</span></span><br><span class="line">    <span class="comment">&lt;!--关联对象property 关联对象在Student实体类中的属性--&gt;</span></span><br><span class="line">    <span class="tag">&lt;<span class="name">association</span> <span class="attr">property</span>=<span class="string">"teacher"</span> <span class="attr">javaType</span>=<span class="string">"Teacher"</span>&gt;</span></span><br><span class="line">        <span class="tag">&lt;<span class="name">result</span> <span class="attr">property</span>=<span class="string">"name"</span> <span class="attr">column</span>=<span class="string">"tname"</span>/&gt;</span></span><br><span class="line">    <span class="tag">&lt;/<span class="name">association</span>&gt;</span></span><br><span class="line"><span class="tag">&lt;/<span class="name">resultMap</span>&gt;</span></span><br></pre></td></tr></table></figure>

<ol start="3">
<li><p>去mabits-config文件中注入</p>
</li>
<li><p>测试</p>
</li>
</ol>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">@Test</span></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">testGetStudents2</span><span class="params">()</span></span>&#123;</span><br><span class="line">    SqlSession session = MybatisUtils.getSession();</span><br><span class="line">    StudentMapper mapper = session.getMapper(StudentMapper<span class="class">.<span class="keyword">class</span>)</span>;</span><br><span class="line"></span><br><span class="line">    List&lt;Student&gt; students = mapper.getStudents2();</span><br><span class="line"></span><br><span class="line">    <span class="keyword">for</span> (Student student : students)&#123;</span><br><span class="line">        System.out.println(</span><br><span class="line">                <span class="string">"学生名:"</span>+ student.getName()</span><br><span class="line">                        +<span class="string">"\t老师:"</span>+student.getTeacher().getName());</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>



<h4 id="小结"><a href="#小结" class="headerlink" title="小结"></a>小结</h4><ul>
<li>按照查询进行嵌套就像SQL中的子查询</li>
<li>按照结果进行嵌套就想SQL中的联表查询</li>
</ul>
<h2 id="一对多处理"><a href="#一对多处理" class="headerlink" title="一对多处理"></a>一对多处理</h2><p>一对多的理解：</p>
<ul>
<li>一个老师拥有多个学生</li>
<li>对于老师这边，就是一个一个一对多的现象，即从一个老师下面拥有一群学生（集合）</li>
<li>collection集合</li>
</ul>
<h3 id="搭建测试环境-1"><a href="#搭建测试环境-1" class="headerlink" title="搭建测试环境"></a>搭建测试环境</h3><p>实体类的编写：</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">@Data</span></span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Student</span> </span>&#123;</span><br><span class="line">    <span class="keyword">private</span> <span class="keyword">int</span> id;</span><br><span class="line">    <span class="keyword">private</span> String name;</span><br><span class="line">    <span class="keyword">private</span> <span class="keyword">int</span> tid;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">@Data</span> </span><br><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Teacher</span> </span>&#123;</span><br><span class="line">    <span class="keyword">private</span> <span class="keyword">int</span> id;</span><br><span class="line">    <span class="keyword">private</span> String name;</span><br><span class="line">    <span class="comment">//一个老师多个学生</span></span><br><span class="line">    <span class="keyword">private</span> List&lt;Student&gt; students;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h4 id="按照结果嵌套处理-1"><a href="#按照结果嵌套处理-1" class="headerlink" title="按照结果嵌套处理"></a>按照结果嵌套处理</h4><ol>
<li>TeacherMapper接口编写方法</li>
</ol>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//获取指定老师，及老师下的所有学生</span></span><br><span class="line"><span class="function"><span class="keyword">public</span> Teacher <span class="title">getTeacher</span><span class="params">(<span class="keyword">int</span> id)</span></span>;</span><br></pre></td></tr></table></figure>

<ol start="2">
<li>编写Mapper配置文件</li>
</ol>
<figure class="highlight xml"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">&lt;?xml version="1.0" encoding="UTF-8" ?&gt;</span></span><br><span class="line"><span class="meta">&lt;!DOCTYPE <span class="meta-keyword">mapper</span></span></span><br><span class="line"><span class="meta">        <span class="meta-keyword">PUBLIC</span> <span class="meta-string">"-//mybatis.org//DTD Mapper 3.0//EN"</span></span></span><br><span class="line"><span class="meta">        <span class="meta-string">"http://mybatis.org/dtd/mybatis-3-mapper.dtd"</span>&gt;</span></span><br><span class="line"><span class="tag">&lt;<span class="name">mapper</span> <span class="attr">namespace</span>=<span class="string">"com.zhuuu.dao.TeacherMapper"</span>&gt;</span></span><br><span class="line"></span><br><span class="line"><span class="comment">&lt;!--思路：</span></span><br><span class="line"><span class="comment">    1.从学生表和老师表中查出学生id,学生姓名，老师姓名</span></span><br><span class="line"><span class="comment">    2.从查询出来的结果上做结果集映射</span></span><br><span class="line"><span class="comment">      1.集合的话，使用collection</span></span><br><span class="line"><span class="comment">      2.javaType和ofType都是用来指定对象类型的</span></span><br><span class="line"><span class="comment">      3.JavaType是用来指定pojo中属性的类型</span></span><br><span class="line"><span class="comment">      4.oftype是用来以后能干涉到list集合属性中pojo的类型--&gt;</span></span><br><span class="line"></span><br><span class="line">    <span class="tag">&lt;<span class="name">select</span> <span class="attr">id</span>=<span class="string">"getTeacher"</span> <span class="attr">resultMap</span>=<span class="string">"TeacherStudent"</span>&gt;</span></span><br><span class="line">        select s.id sid, s.name sname,t.name tname,t.id tid</span><br><span class="line">        from student s,teacher t</span><br><span class="line">        where s.tid = t.id and t.id=#&#123;id&#125;</span><br><span class="line">    <span class="tag">&lt;/<span class="name">select</span>&gt;</span></span><br><span class="line"></span><br><span class="line"></span><br><span class="line">    <span class="tag">&lt;<span class="name">resultMap</span> <span class="attr">id</span>=<span class="string">"TeacherStudent"</span> <span class="attr">type</span>=<span class="string">"Teacher"</span>&gt;</span></span><br><span class="line">        <span class="tag">&lt;<span class="name">result</span> <span class="attr">property</span>=<span class="string">"name"</span> <span class="attr">column</span>=<span class="string">"tname"</span>/&gt;</span></span><br><span class="line">        <span class="tag">&lt;<span class="name">collection</span> <span class="attr">property</span>=<span class="string">"students"</span> <span class="attr">ofType</span>=<span class="string">"Student"</span>&gt;</span></span><br><span class="line">            <span class="tag">&lt;<span class="name">result</span> <span class="attr">property</span>=<span class="string">"id"</span> <span class="attr">column</span>=<span class="string">"sid"</span>/&gt;</span></span><br><span class="line">            <span class="tag">&lt;<span class="name">result</span> <span class="attr">property</span>=<span class="string">"name"</span> <span class="attr">column</span>=<span class="string">"sname"</span>/&gt;</span></span><br><span class="line">            <span class="tag">&lt;<span class="name">result</span> <span class="attr">property</span>=<span class="string">"tid"</span> <span class="attr">column</span>=<span class="string">"tid"</span>/&gt;</span></span><br><span class="line">        <span class="tag">&lt;/<span class="name">collection</span>&gt;</span></span><br><span class="line">    <span class="tag">&lt;/<span class="name">resultMap</span>&gt;</span></span><br><span class="line"><span class="tag">&lt;/<span class="name">mapper</span>&gt;</span></span><br></pre></td></tr></table></figure>

<ol start="3">
<li>将Mapper文件注册到MyBatis-config文件中</li>
<li>测试</li>
</ol>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">@Test</span></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">testGetTeacher</span><span class="params">()</span></span>&#123;</span><br><span class="line">    SqlSession session = MybatisUtils.getSession();</span><br><span class="line">    TeacherMapper mapper = session.getMapper(TeacherMapper<span class="class">.<span class="keyword">class</span>)</span>;</span><br><span class="line">    Teacher teacher = mapper.getTeacher(<span class="number">1</span>);</span><br><span class="line">    System.out.println(teacher.getName());</span><br><span class="line">    System.out.println(teacher.getStudents());</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h4 id="按照嵌套处理"><a href="#按照嵌套处理" class="headerlink" title="按照嵌套处理"></a>按照嵌套处理</h4><ol>
<li>TeacherMapper接口编写方法</li>
</ol>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> Teacher <span class="title">getTeacher2</span><span class="params">(<span class="keyword">int</span> id)</span></span>;</span><br></pre></td></tr></table></figure>

<ol start="2">
<li>编写接口对应的Mapper配置文件</li>
</ol>
<figure class="highlight xml"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="tag">&lt;<span class="name">select</span> <span class="attr">id</span>=<span class="string">"getTeacher2"</span> <span class="attr">resultMap</span>=<span class="string">"TeacherStudent2"</span>&gt;</span></span><br><span class="line">  select * from teacher where id = #&#123;id&#125;</span><br><span class="line"><span class="tag">&lt;/<span class="name">select</span>&gt;</span></span><br><span class="line"><span class="tag">&lt;<span class="name">resultMap</span> <span class="attr">id</span>=<span class="string">"TeacherStudent2"</span> <span class="attr">type</span>=<span class="string">"Teacher"</span>&gt;</span></span><br><span class="line">    <span class="comment">&lt;!--column是一对多的外键 , 写的是一的主键的列名--&gt;</span></span><br><span class="line">    <span class="tag">&lt;<span class="name">collection</span> <span class="attr">property</span>=<span class="string">"students"</span> <span class="attr">javaType</span>=<span class="string">"ArrayList"</span> <span class="attr">ofType</span>=<span class="string">"Student"</span> <span class="attr">column</span>=<span class="string">"id"</span> <span class="attr">select</span>=<span class="string">"getStudentByTeacherId"</span>/&gt;</span></span><br><span class="line"><span class="tag">&lt;/<span class="name">resultMap</span>&gt;</span></span><br><span class="line"><span class="tag">&lt;<span class="name">select</span> <span class="attr">id</span>=<span class="string">"getStudentByTeacherId"</span> <span class="attr">resultType</span>=<span class="string">"Student"</span>&gt;</span></span><br><span class="line">    select * from student where tid = #&#123;id&#125;</span><br><span class="line"><span class="tag">&lt;/<span class="name">select</span>&gt;</span></span><br></pre></td></tr></table></figure>

<ol start="3">
<li><p>将Mapper文件注册到MyBatis-config文件中</p>
</li>
<li><p>测试</p>
</li>
</ol>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">@Test</span></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">testGetTeacher2</span><span class="params">()</span></span>&#123;</span><br><span class="line">    SqlSession session = MybatisUtils.getSession();</span><br><span class="line">    TeacherMapper mapper = session.getMapper(TeacherMapper<span class="class">.<span class="keyword">class</span>)</span>;</span><br><span class="line">    Teacher teacher = mapper.getTeacher2(<span class="number">1</span>);</span><br><span class="line">    System.out.println(teacher.getName());</span><br><span class="line">    System.out.println(teacher.getStudents());</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h2 id="小结-1"><a href="#小结-1" class="headerlink" title="小结"></a>小结</h2><ul>
<li>关联-associtaion</li>
<li>集合-collection</li>
<li>association是用于一对一和多对一</li>
<li>collection是用于一对多的关系</li>
</ul>
<ul>
<li>JavaType和ofType都是用来指定对象类型的<ul>
<li>JavaType是用来指定pojo中的类型    （说白了就是返回的类名）</li>
<li>ofType到指定的是映射到list集合属性中pojo的类型  （说白了就是泛型）</li>
</ul>
</li>
</ul>

      
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